Primary Treatment of Wastewater: Removing Solids and Preparing for Advanced Treatment

Primary treatment of wastewater is the second stage of wastewater treatment and follows preliminary treatment.

The primary treatment of wastewater involves the physical removal of solid materials and organic matter from the wastewater.

This stage of treatment typically involves physical processes such as sedimentation and/or flotation. Here is a more detailed explanation of the primary treatment process:

Sedimentation

Sedimentation is a process that takes advantage of the force of gravity to separate solids from wastewater. In primary wastewater treatment, the sedimentation process typically occurs in a large tank, called a sedimentation tank, where wastewater is introduced and allowed to settle. The sedimentation tank can be rectangular or circular in shape, and its size and design can vary depending on the specific needs of the wastewater treatment facility.

The sedimentation tank is designed to reduce the velocity of the wastewater, which allows the heavier solids, such as sand, grit, and organic matter, to settle to the bottom of the tank. The rate of settling is influenced by the size, shape, and density of the particles in the wastewater, as well as the temperature and composition of the water.

During the sedimentation process, the wastewater enters the tank and is distributed evenly across the surface of the tank. As the water flows through the tank, the heavier particles settle to the bottom and form a layer of sludge, while the lighter particles rise to the top and form a layer of clarified water. The clarified water is then removed from the top of the tank and sent for further treatment, while the sludge is removed from the bottom of the tank and treated separately.

The sedimentation process in primary wastewater treatment is effective at removing a significant portion of the suspended solids, organic matter, and other large particles from the wastewater. However, it is not effective at removing dissolved pollutants, such as nitrogen and phosphorus, or other contaminants, such as bacteria and viruses. Therefore, after primary treatment, the wastewater typically undergoes secondary treatment, which uses biological and chemical processes to further treat the water and remove the remaining contaminants.

Flotation

Flotation is an alternative process to sedimentation in primary wastewater treatment that uses air to float solids to the surface of the water, where they can be removed. This process is also known as Dissolved Air Flotation (DAF), as it involves dissolving air in the wastewater to form bubbles, which attach to the solids and float them to the surface.

Steps of flotation process in primary treatment of sewage

• The wastewater is first treated with a coagulant or flocculant to promote the aggregation of solids into larger particles.
• Next, the wastewater is introduced into a flotation tank, where the water is saturated with air under pressure.
• The air is then released from the water, forming small bubbles that attach to the solids in the wastewater, causing them to float to the surface.
• As the solids rise to the surface, they form a layer of sludge that can be skimmed off the top of the water.
• The clarified water is then removed from the bottom of the tank and sent for further treatment.
• The sludge removed from the surface can then be sent to a separate treatment process to remove any remaining solids.

The flotation process is effective at removing fine particles and light, floating solids that are difficult to remove using sedimentation. It can also be used to remove fats, oils, and greases from the wastewater. Additionally, the process can improve the efficiency of downstream treatment processes by reducing the load of suspended solids and organic matter.

However, flotation can be more complex and expensive to operate compared to sedimentation, as it requires the use of specialized equipment to dissolve air in the wastewater and remove the solids from the surface. Therefore, the choice of sedimentation or flotation for primary wastewater treatment often depends on the specific requirements of the wastewater treatment facility and the characteristics of the wastewater being treated.

Effluent Quality: After primary treatment of wastewater

Effluent quality refers to the quality of the treated wastewater that is discharged from a primary wastewater treatment plant. The quality of the effluent can vary depending on several factors, including the design and operation of the treatment plant, the quality of the influent wastewater, and the treatment processes used.

In primary wastewater treatment, the effluent quality is typically measured by several parameters.

Parameters to measure effluent quality

1. Suspended solids: The concentration of suspended solids in the effluent can indicate how effectively the sedimentation or flotation process has removed solids from the wastewater. The concentration of suspended solids is typically measured in milligrams per liter (mg/L).
2. Biochemical Oxygen Demand (BOD): BOD is a measure of the amount of organic matter in the wastewater, which can indicate how well the treatment plant is removing organic pollutants. BOD is typically measured in milligrams of oxygen per liter of wastewater (mg/L).
3. Total nitrogen and phosphorus: These nutrients are often regulated in wastewater discharge permits, as they can contribute to eutrophication and other environmental problems in receiving waters.
4. pH: The pH of the effluent can affect the ability of microorganisms to break down pollutants and can also impact the potential for corrosion and scaling in wastewater infrastructure.
5. Pathogens: The presence of pathogens, such as bacteria and viruses, in the effluent can pose a risk to public health and the environment.

The effluent quality requirements for a specific wastewater treatment plant can vary depending on the discharge permit issued by the regulatory agency overseeing the plant. The permit typically specifies the maximum allowable concentration of various pollutants in the effluent, and the treatment plant must operate within these limits to avoid fines or other penalties.

To achieve the required effluent quality, primary wastewater treatment may be followed by additional treatment processes, such as secondary treatment, which can further remove pollutants and improve the quality of the effluent. Primary treatment is not designed to remove dissolved pollutants, pathogens, or nutrients. However, it can remove up to 60% of suspended solids and 30% of biological oxygen demand (BOD) from the wastewater. Additionally, treatment plants may use advanced technologies, such as ultraviolet disinfection, to remove pathogens from the effluent before it is discharged into the environment.

Design and operation of primary clarifiers

The design and operation of primary clarifiers, or sedimentation tanks, is crucial to achieving effective primary wastewater treatment. The design of these tanks depends on several factors, including the flow rate of the wastewater, the solids content of the wastewater, and the size and type of the treatment plant. Here is some more detailed information about the design and operation of primary clarifiers:

Design of primary clarifiers

1. Tank shape and size: The shape and size of the tank are important factors that affect the efficiency of the clarifier. Rectangular tanks are more commonly used, but circular tanks are also used in some facilities. The tank’s size should be large enough to provide sufficient retention time for the wastewater to allow for the settling of solids.
2. Inlet and outlet design: The inlet structure is designed to distribute the wastewater evenly across the surface of the tank. The outlet structure should be located at the opposite end of the tank from the inlet to allow the clarified water to be removed without disturbing the settled solids.
3. Baffles: Baffles are used to promote uniform flow and to prevent short-circuiting in the tank.
4. Sludge removal: A sludge removal mechanism, such as a scraper, is used to remove the settled solids from the bottom of the tank.

Operation of primary clarifiers

1. Wastewater flow rate: The flow rate of wastewater into the clarifier should be controlled to prevent short-circuiting and ensure that the solids have sufficient time to settle.
2. Retention time: The retention time in the clarifier should be optimized to allow for maximum settling of solids.
3. Sludge removal: The settled solids, or sludge, should be removed from the bottom of the tank regularly to prevent accumulation.
4. Inlet control: The inlet structure should be designed to provide even distribution of the wastewater across the surface of the tank.
5. Baffle maintenance: Baffles should be inspected regularly to ensure they are functioning correctly and are in good condition.
6. Clarifier performance: The performance of the clarifier should be monitored regularly by measuring influent and effluent flow rates, and concentrations of suspended solids, to ensure the treatment plant is meeting effluent quality requirements.

Overall, the design and operation of primary clarifiers are critical to achieving efficient primary wastewater treatment. Proper design and operation can help maximize the removal of suspended solids and other pollutants from the wastewater. It leads to improve effluent quality and environmental protection.

In summary, preliminary and primary wastewater treatment are critical stages in the overall wastewater treatment process. Preliminary treatment removes large and bulky materials from the wastewater. The primary treatment of wastewater involves the physical removal of solid materials and organic matter from the wastewater. It reduces the overload on downstream processes and improves the efficiency of the treatment process. These processes set the stage for the more advanced secondary and tertiary treatment processes. Which are designed to remove dissolved pollutants, pathogens, and nutrients.

FAQs

1. What is the primary treatment of wastewater?

Primary treatment of wastewater is the second stage of wastewater treatment that involves the physical removal of solid materials and organic matter from the wastewater.

2. What are the physical processes involved in the primary treatment of wastewater?

The physical processes involved in the primary treatment of wastewater are sedimentation and/or flotation.

3. What is sedimentation?

Sedimentation is a process that takes advantage of the force of gravity to separate solids from wastewater. In primary wastewater treatment, the sedimentation process typically occurs in a large tank, called a sedimentation tank, where wastewater is introduced and allowed to settle.

4. What is flotation?

Flotation is an alternative process to sedimentation in primary wastewater treatment that uses air to float solids to the surface of the water, where they can be removed. This process is also known as Dissolved Air Flotation (DAF), as it involves dissolving air in the wastewater to form bubbles, which attach to the solids and float them to the surface.

5. What are the steps of the flotation process in the primary treatment of sewage?

The steps of the flotation process in the primary treatment of sewage are: treating the wastewater with a coagulant or flocculant, introducing the wastewater into a flotation tank, saturating the water with air under pressure, releasing the air from the water to form bubbles, allowing the solids to float to the surface to form a layer of sludge, skimming the sludge off the top of the water, and removing the clarified water from the bottom of the tank for further treatment.

6. What are the parameters used to measure effluent quality?

The parameters used to measure effluent quality are suspended solids, Biochemical Oxygen Demand (BOD), total nitrogen and phosphorus, pH, and pathogens.

7. What is the effluent quality?

Effluent quality refers to the quality of the treated wastewater that is discharged from a primary wastewater treatment plant.

Read Also:

Wastewater Treatment Process Overview

Preliminary Treatment of Wastewater

Fluid Flow Operations